Cubic boron nitride sintered material and method of producing same

ABSTRACT

A cubic boron nitride sintered material includes: 0 to 85 volume % of cubic boron nitride grains; and a binder phase, wherein the binder phase includes at least one selected from a group consisting of one or more first compounds and a solid solution originated from the first compounds, the cubic boron nitride grains include, on number basis, more than or equal to 50% of cubic boron nitride grains each having an equivalent circle diameter of more than 0.5 μm, and includes, on number basis, less than or equal to 50% of cubic boron nitride grains each having an equivalent circle diameter of more than 2 μm, and when a mass of the cubic boron nitride grains is assumed as 100 mass %, a total content of lithium, magnesium, calcium, strontium, beryllium, and barium in the cubic boron nitride grains is less than 0.001 mass %.

TECHNICAL FIELD

The present disclosure relates to a cubic boron nitride sinteredmaterial and a method of producing the cubic boron nitride sinteredmaterial. The present application claims a priority based on JapanesePatent Application No. 2019-226360 filed on Dec. 16, 2019, the entirecontent of which is incorporated herein by reference.

BACKGROUND ART

A cubic boron nitride (hereinafter, also referred to as “cBN”) sinteredmaterial has very high hardness and has excellent thermal stability andchemical stability, and is therefore used in a cutting tool or awear-resistant tool.

Each of Japanese Patent Laying-Open No. 2005-187260 (PTL 1) and WO2005/066381 (PTL 2) discloses a method of obtaining a cubic boronnitride including cBN grains and a binder by mixing a cubic boronnitride powder and a binder powder to obtain a powder mixture andsintering the powder mixture under an ultra-high pressure and hightemperature condition.

Each of Japanese Patent Laying-Open No. 2015-202981 (PTL 3) and JapanesePatent Laying-Open No. 2015-202980 (PTL 4) discloses a method ofobtaining a cubic boron nitride composite sintered material including acubic boron nitride polycrystalline material and a ceramic phase bymixing normal pressure type boron nitride and a ceramic to obtain amixture and sintering the mixture under an ultra-high pressure and hightemperature condition.

CITATION LIST Patent Literature

PTL 1: Japanese Patent Laying-Open No. 2005-187260

PTL 2: WO 2005/066381

PTL 3: Japanese Patent Laying-Open No. 2015-202981

PTL 4: Japanese Patent Laying-Open No. 2015-202980

SUMMARY OF INVENTION

A cubic boron nitride sintered material according to the presentdisclosure is a cubic boron nitride sintered material including: morethan or equal to 40 volume % and less than or equal to 85 volume % ofcubic boron nitride grains; and a binder phase, wherein

the binder phase includes at least one selected from a group consistingof one or more first compounds and a solid solution originated from thefirst compounds, or includes at least one selected from a groupconsisting of the one or more first compounds and the solid solutionoriginated from the first compounds and an aluminum compound, the one ormore first compounds consisting of at least one element selected from agroup consisting of titanium, zirconium, hafnium, vanadium, niobium,tantalum, chromium, molybdenum, and tungsten, and at least one elementselected from a group consisting of nitrogen, carbon, boron, and oxygen,

the cubic boron nitride grains include, on number basis, more than orequal to 50% of cubic boron nitride grains each having an equivalentcircle diameter of more than 0.5 μm, and includes, on number basis, lessthan or equal to 50% of cubic boron nitride grains each having anequivalent circle diameter of more than 2 μm, and

when a mass of the cubic boron nitride grains is assumed as 100 mass %,a total content of lithium, magnesium, calcium, strontium, beryllium,and barium in the cubic boron nitride grains is less than 0.001 mass %.

A method of producing a cubic boron nitride sintered material accordingto the present disclosure is a method of producing the above-describedcubic boron nitride sintered material, the method including:

obtaining a powder mixture by mixing a hexagonal boron nitride powderand a binder powder; and

obtaining the cubic boron nitride sintered material by sintering thepowder mixture by increasing a pressure to more than or equal to 8 GPaand less than or equal to 20 GPa, increasing a temperature to more thanor equal to 2300° C. and less than or equal to 2500° C., and holding,for more than or equal to 30 minutes and less than 90 minutes, thepowder mixture at maximum pressure and maximum temperature reached byincreasing the pressure and the temperature.

DETAILED DESCRIPTION

[Problem to be Solved by the Present Disclosure]

Each of tools employing the cubic boron nitride sintered materials ofPTL 1 and PTL 2 has been required to attain further improved toolperformance such as wear resistance and breakage resistance particularlywhen used in high-load processing of hardened steel. As a result ofreviewing mechanisms of occurrence of wear and breakage in each of thetools employing the cubic boron nitride sintered materials of PTL 1 andPTL 2, the present inventors have newly anticipated the followingmechanism.

In each of PTL 1 and PTL 2, the cubic boron nitride powder is used as asource material. The cubic boron nitride powder is produced by treatinghexagonal boron nitride (hereinafter, also referred to as “hBN”) and acatalyst under high temperature and high pressure that are thermalstability conditions for cBN. As the catalyst, an alkali metal element(lithium), an alkaline earth metal element (magnesium, calcium,strontium, beryllium, or barium), or the like is generally used.Therefore, the obtained cubic boron nitride powder includes the catalystelement.

When cutting is performed using a tool employing a cubic boron nitridesintered material, pressure and temperature of the tool in the vicinityof a contact point with a workpiece are increased. Particularly, in thecase of high-load processing of hardened steel, the pressure andtemperature are significantly increased. When the cubic boron nitridesintered material includes the catalyst element, the catalyst elementpromotes phase conversion from cubic boron nitride to hexagonal boronnitride under the condition of the pressure and temperature of the toolin the vicinity of the contact point with the workpiece in the case ofthe high-load processing of hardened steel. Therefore, thermalconductivity or hardness tends to be decreased in the vicinity of thecontact point of the cutting edge of the tool with the workpiece. Thedecreased thermal conductivity and decreased hardness are considered tocause a decrease in cutting performance such as wear resistance andbreakage resistance.

Each of tools employing the cubic boron nitride composite sinteredmaterials of PTL 3 and PTL 4 has been also required to attain furtherimproved tool performance such as wear resistance and breakageresistance particularly when used in high-load processing of hardenedsteel. As a result of reviewing mechanisms of occurrence of wear andbreakage in each of the tools employing the cubic boron nitridecomposite sintered materials of PTL 3 and PTL 4, the present inventorshave newly anticipated the following mechanism.

In each of PTL 3 and PTL 4, the cubic boron nitride single crystal inthe cubic boron nitride polycrystalline material has an small averagecrystal grain size of less than or equal to 500 nm and therefore hasfine grains. When a large amount of the fine grain component exists inthe cubic boron nitride sintered material, toughness and thermalconductivity of the cubic boron nitride sintered material tend to bedecreased. This is considered to cause a decrease in tool performancesuch as wear resistance and breakage resistance particularly in thehigh-load processing of hardened steel.

Based on the newly anticipated mechanism, the present inventorshypothesized that the tool performance such as wear resistance andbreakage resistance is affected by the amount of the catalyst element inthe cubic boron nitride sintered material and the grain sizes of thecubic boron nitride single crystal.

The present disclosure has been obtained by the present inventors as aresult of diligent study based on the anticipated new mechanism and thehypothesis.

It is an object of the present disclosure to provide a cubic boronnitride sintered material that can have excellent cutting performanceeven in high-load processing of hardened steel when used as a tool.

[Advantageous Effect of the Present Disclosure]

The cubic boron nitride sintered material according to the presentdisclosure can have excellent cutting performance even in high-loadprocessing of hardened steel when used as a tool.

[Description of Embodiments]

First, embodiments of the present disclosure are listed and described.

(1) A cubic boron nitride sintered material according to the presentdisclosure is a cubic boron nitride sintered material including: morethan or equal to 40 volume % and less than or equal to 85 volume % ofcubic boron nitride grains; and a binder phase, wherein

the binder phase includes at least one selected from a group consistingof one or more first compounds and a solid solution originated from thefirst compounds, or includes at least one selected from a groupconsisting of the one or more first compounds and the solid solutionoriginated from the first compounds and an aluminum compound, the one ormore first compounds consisting of at least one element selected from agroup consisting of titanium, zirconium, hafnium, vanadium, niobium,tantalum, chromium, molybdenum, and tungsten, and at least one elementselected from a group consisting of nitrogen, carbon, boron, and oxygen,

the cubic boron nitride grains include, on number basis, more than orequal to 50% of cubic boron nitride grains each having an equivalentcircle diameter of more than 0.5 μm, and includes, on number basis, lessthan or equal to 50% of cubic boron nitride grains each having anequivalent circle diameter of more than 2 μm, and

when a mass of the cubic boron nitride grains is assumed as 100 mass %,a total content of lithium, magnesium, calcium, strontium, beryllium,and barium in the cubic boron nitride grains is less than 0.001 mass %.

The cubic boron nitride sintered material according to the presentdisclosure can have excellent cutting performance even in high-loadprocessing of hardened steel when used as a tool.

(2) In an X-ray diffraction spectrum of the cubic boron nitride sinteredmaterial, a relation of the following formula I is preferably indicated:

(IA+IB+IC)/ID≤0.05  formula I, where

IA represents a peak intensity originated from compressed hexagonalboron nitride, IB represents a peak intensity originated from hexagonalboron nitride, IC represents a peak intensity originated from wurtzitetype boron nitride, and ID represents a peak intensity originated fromcubic boron nitride.

Thus, the breakage resistance of the tool using the cubic boron nitrideis further improved.

(3) A method of producing a cubic boron nitride sintered materialaccording to the present disclosure is a method of producing theabove-described cubic boron nitride sintered material, the methodincluding:

obtaining a powder mixture by mixing a hexagonal boron nitride powderand a binder powder; and

obtaining the cubic boron nitride sintered material by sintering thepowder mixture by increasing a pressure to more than or equal to 8 GPaand less than or equal to 20 GPa, increasing a temperature to more thanor equal to 2300° C. and less than or equal to 2500° C., and holding,for more than or equal to 30 minutes and less than 90 minutes, thepowder mixture at maximum pressure and maximum temperature reached byincreasing the pressure and the temperature.

Thus, the cubic boron nitride sintered material can be obtained whichcan have excellent cutting performance even in high-load processing ofhardened steel when used as a tool.

[Details of Embodiments of the Present Disclosure]

The following describes a cubic boron nitride sintered material and amethod of producing the cubic boron nitride sintered material accordingto the present disclosure.

When a compound or the like is expressed by a chemical formula in thepresent specification and an atomic ratio is not particularly limited,it is assumed that all the conventionally known atomic ratios areincluded. The atomic ratio should not be necessarily limited only to onein the stoichiometric range. For example, when “TiN” is described, anatomic ratio in the TiN include all the conventionally known atomicratios. The same also applies to compounds other than the “TiN”.

First Embodiment Cubic Boron Nitride Sintered Material

A cubic boron nitride sintered material according to the presentdisclosure is a cubic boron nitride sintered material including: morethan or equal to 40 volume % and less than or equal to 85 volume % ofcubic boron nitride grains; and a binder phase, wherein the binder phaseincludes at least one selected from a group consisting of one or morefirst compounds and a solid solution originated from the firstcompounds, or includes at least one selected from a group consisting ofthe one or more first compounds and the solid solution originated fromthe first compounds and an aluminum compound, the one or more firstcompounds consisting of at least one element selected from a groupconsisting of titanium, zirconium, hafnium, vanadium, niobium, tantalum,chromium, molybdenum, and tungsten, and at least one element selectedfrom a group consisting of nitrogen, carbon, boron, and oxygen, thecubic boron nitride grains include, on number basis, more than or equalto 50% of cubic boron nitride grains each having an equivalent circlediameter of more than 0.5 μm, and includes, on number basis, less thanor equal to 50% of cubic boron nitride grains each having an equivalentcircle diameter of more than 2 μm, and when a mass of the cubic boronnitride grains is assumed as 100 mass %, a total content of lithium,magnesium, calcium, strontium, beryllium, and barium in the cubic boronnitride grains is less than 0.001 mass %.

The cubic boron nitride sintered material according to the presentdisclosure can have excellent cutting performance even in high-loadprocessing of hardened steel when used as a tool. This is presumably dueto the following reasons (i) to (iv).

(i) The cubic boron nitride sintered material according to the presentdisclosure includes more than or equal to 40 volume % and less than orequal to 85 volume % of the cubic boron nitride grains having excellentstrength and toughness. Thus, the cubic boron nitride sintered materialcan also have excellent strength and toughness. Therefore, a toolemploying the cubic boron nitride sintered material can have excellentwear resistance and breakage resistance even in high-load processing ofhardened steel.

(ii) In the cubic boron nitride sintered material according to thepresent disclosure, the binder phase includes at least one selected froma group consisting of one or more first compounds and a solid solutionoriginated from the first compounds, or includes at least one selectedfrom a group consisting of the one or more first compounds and the solidsolution originated from the first compounds and an aluminum compound,the one or more first compounds consisting of at least one elementselected from a group consisting of titanium, zirconium, hafnium,vanadium, niobium, tantalum, chromium, molybdenum, and tungsten, and atleast one element selected from a group consisting of nitrogen, carbon,boron, and oxygen. Each of the first compounds has high strength andtoughness itself and serves to improve binding force between the cBNgrains. Therefore, a tool employing the cubic boron nitride sinteredmaterial including the first compound(s) as a binder phase can haveexcellent wear resistance and breakage resistance even in high-loadprocessing of hardened steel.

(iii) In the cubic boron nitride sintered material according to thepresent disclosure, the cubic boron nitride grains include, on numberbasis, more than or equal to 50% of cubic boron nitride grains eachhaving an equivalent circle diameter of more than 0.5 μm, and includes,on number basis, less than or equal to 50% of cubic boron nitride grainseach having an equivalent circle diameter of more than 2 μm.

When the cubic boron nitride grains are fine grains, the toughness andthermal conductivity of the cubic boron nitride sintered material tendto be decreased. In the cubic boron nitride sintered material accordingto the present disclosure, the cubic boron nitride grains each having anequivalent circle diameter of more than 0.5 μm are more than or equal to50% on number basis. That is, since the ratio of the cubic boron nitridegrains each having an equivalent circle diameter of less than or equalto 0.5 μm, i.e., the ratio of the fine grains, is less than or equal to50% and therefore the ratio of the fine grains is small, the cubic boronnitride sintered material can have excellent toughness and thermalconductivity.

When the cubic boron nitride grains are coarse grains, the strength ofthe cubic boron nitride sintered material tends to be decreased. In thecubic boron nitride sintered material according to the presentdisclosure, since the ratio of the cubic boron nitride grains eachhaving an equivalent circle diameter of more than 2 μm, i.e., the ratioof the coarse grains, is less than or equal to 50% and therefore theratio of the coarse grains is small, the cubic boron nitride sinteredmaterial can have excellent strength.

Therefore, the tool employing the cubic boron nitride sintered materialaccording to the present disclosure can have excellent wear resistanceand breakage resistance even in the high-load processing of hardenedsteel.

(iv) In the cubic boron nitride sintered material according to thepresent disclosure, when a mass of the cubic boron nitride grains isassumed as 100 mass %, a total content of lithium, magnesium, calcium,strontium, beryllium, and barium (hereinafter, these elements are alsoreferred to as “catalyst elements”) in the cubic boron nitride grains isless than 0.001 mass %. When the catalyst elements are present in thecubic boron nitride grains, the catalyst elements promote phaseconversion from cubic boron nitride to hexagonal boron nitride under acondition of pressure and temperature in the vicinity of a contact pointbetween the tool and a workpiece in the high-load processing of hardenedsteel. Therefore, thermal conductivity and hardness tend to be decreasedin the vicinity of the contact point of the cutting edge of the toolwith the workpiece.

In the cubic boron nitride sintered material according to the presentdisclosure, since the total content of the catalyst elements in thecubic boron nitride grains is less than 0.001 mass %, the phaseconversion from cubic boron nitride to hexagonal boron nitride by thecatalyst elements is less likely to occur. Therefore, the tool employingthe cubic boron nitride sintered material according to the presentdisclosure can have excellent wear resistance and breakage resistanceeven in the high-load processing of hardened steel.

<Composition>

The cubic boron nitride sintered material according to the presentdisclosure includes: more than or equal to 40 volume % and less than orequal to 85 volume % of the cubic boron nitride grains; and the binderphase.

Since the cubic boron nitride sintered material according to the presentdisclosure includes the cubic boron nitride having excellent strengthand toughness, the cubic boron nitride sintered material can also haveexcellent strength and toughness. Therefore, the tool employing thecubic boron nitride sintered material can have excellent wear resistanceand breakage resistance even in the high-load processing of hardenedsteel.

The lower limit of the content ratio of the cBN grains in the cBNsintered material is 40 volume %, and is preferably 45 volume %. Theupper limit of the content ratio of the cBN grains in the cBN sinteredmaterial is 85 volume %, and is preferably 75 volume %. The contentratio of the cBN grains in the cBN sintered material is preferably morethan or equal to 45 volume % and less than or equal to 75 volume %.

Since the cubic boron nitride sintered material according to the presentdisclosure has high strength and includes the binder phase that servesto improve binding force between the cBN grains, the cubic boron nitridesintered material can also have excellent strength and toughness.Therefore, the tool employing the cubic boron nitride sintered materialcan have excellent wear resistance and breakage resistance even in thehigh-load processing of hardened steel.

The lower limit of the content ratio of the binder phase in the cBNsintered material is preferably 15 volume %, and is more preferably 25volume %. The upper limit of the content ratio of the binder phase inthe cBN sintered material is preferably 60 volume %, and is morepreferably 55 volume %. The content ratio of the binder phase in the cBNsintered material is preferably more than or equal to 15 volume % andless than or equal to 60 volume %, and is more preferably more than orequal to 25 volume % and less than or equal to 55 volume %.

The content ratio (volume %) of the cBN grains and the content ratio(volume %) of the binder phase in the cBN sintered material can bechecked by performing structure observation, elemental analysis, and thelike onto the cBN sintered material using an energy dispersive X-rayanalysis device (EDX) (“Octane Elect EDS system” (trademark) provided byEDAX) accompanied with a scanning electron microscope (SEM) (“JSM-7800F”(trademark) provided by JEOL).

Specifically, the content ratio (volume %) of the cBN grains can becalculated as follows. First, the cBN sintered material is cut at anarbitrary location to produce a sample including a cross section of thecBN sintered material. For the formation of the cross section, a focusedion beam device, a cross section polisher device, or the like can beused. Next, the cross section is observed by the SEM at a magnificationof 5000× to obtain a reflected electron image. In the reflected electronimage, the cBN grains look black (dark fields) and a region having thebinder phase existing therein is gray or white (bright fields).

Next, the reflected electron image is subjected to binarizationprocessing using image analysis software (for example, “WinROOF”provided by Mitani Corporation). From the image having been through thebinarization processing, the area ratio of pixels originated from darkfields (pixels originated from the cBN grains) in the area of themeasurement visual field is calculated. The calculated area ratio isregarded as volume %, thereby finding the content ratio (volume %) ofthe cBN grains.

From the image having been through the binarization processing, the arearatio of pixels originated from bright fields (pixels originated fromthe binder phase) in the area of the measurement visual field iscalculated, thereby finding the content ratio (volume %) of the binderphase.

The cubic boron nitride sintered material according to the presentdisclosure may include an inevitable impurity as long as the effect ofthe present disclosure is exhibited. Examples of the inevitable impurityinclude tungsten and cobalt. When the cubic boron nitride sinteredmaterial includes such an inevitable impurity, the content of theinevitable impurity is preferably less than or equal to 0.1 mass %. Thecontent of the inevitable impurity can be measured by secondary ion massspectrometry (SIMS).

The cubic boron nitride sintered material according to the presentdisclosure may include at least one of compressed hexagonal boronnitride, hexagonal boron nitride, and wurtzite type boron nitride withinthe scope in which the effects of the present disclosure are exhibited.Here, the “compressed hexagonal boron nitride” refers to a hexagonalboron nitride having a crystal structure similar to the crystalstructure of an ordinary hexagonal boron nitride and having aninterplanar spacing smaller than the interplanar spacing (0.333 nm) ofthe ordinary hexagonal boron nitride in a c-axis direction. Therespective content ratios of the compressed hexagonal boron nitride, thehexagonal boron nitride, and the wurtzite type boron nitride withrespect to the cubic boron nitride will be described in detail in thebelow-described section <X-Ray Diffraction Spectrum>.

<Cubic Boron Nitride Grains>

(Equivalent Circle Diameter)

The cubic boron nitride grains included in the cubic boron nitridesintered material according to the present disclosure include, on numberbasis, more than or equal to 50% of cubic boron nitride grains eachhaving an equivalent circle diameter of more than 0.5 μm, and includes,on number basis, less than or equal to 50% of cubic boron nitride grainseach having an equivalent circle diameter of more than 2 μm. It shouldbe noted that when calculating on number basis, grains each having anequivalent circle diameter of less than 0.05 μm are not counted.

When the ratio of the cubic boron nitride grains each having anequivalent circle diameter of more than 0.5 μm is more than or equal to50%, the cubic boron nitride sintered material can have excellenttoughness and thermal conductivity. When the ratio of the cubic boronnitride grains each having an equivalent circle diameter of more than 2μm is less than or equal to 50%, the cubic boron nitride sinteredmaterial can have excellent strength. Therefore, the tool employing thecubic boron nitride sintered material according to the presentdisclosure can have excellent wear resistance and breakage resistanceeven in the high-load processing of hardened steel.

The lower limit of the number-based ratio of the cubic boron nitridegrains each having an equivalent circle diameter of more than 0.5 μm inthe whole of the cubic boron nitride grains is 50%, and is preferably70%. The upper limit of the number-based ratio of the cubic boronnitride grains each having an equivalent circle diameter of more than0.5 μm in the whole of the cubic boron nitride grains is preferably100%. The number-based ratio of the cubic boron nitride grains eachhaving an equivalent circle diameter of more than 0.5 μm in the whole ofthe cubic boron nitride grains is preferably more than or equal to 50%and less than or equal to 100%, and is more preferably more than orequal to 70% and less than or equal to 100%.

The lower limit of the number-based ratio of the cubic boron nitridegrains each having an equivalent circle diameter of more than 2 μm inthe whole of the cubic boron nitride grains is preferably 0%. The upperlimit of the ratio of the cubic boron nitride grains each having anequivalent circle diameter of more than 2 μm in the whole of the cubicboron nitride grains is 50%, and is preferably 20%. The ratio of thecubic boron nitride grains each having an equivalent circle diameter ofmore than 2 μm in the whole of the cubic boron nitride grains ispreferably more than or equal to 0% and less than or equal to 50%, andis more preferably more than or equal to 0% and less than or equal to20%.

The following specifically describes: a method of calculating thenumber-based ratio of the cubic boron nitride grains each having anequivalent circle diameter of more than 0.5 μm in the cubic boronnitride grains; and a method of calculating the number-based ratio ofthe cubic boron nitride grains each having an equivalent circle diameterof more than 2 μm in the cubic boron nitride grains.

First, the cubic boron nitride sintered material is cut by a diamondgrindstone electrodeposited wire or the like so as to expose measurementpositions, and the cross section thereof is polished. When the cubicboron nitride sintered material is used as a portion of a tool, theportion of the cubic boron nitride sintered material is cut out by thediamond grindstone electrodeposited wire or the like, and the crosssection of the cut-out portion is polished. Five measurement positionsare arbitrarily set on the polished surface. Five SEM images areobtained by observing the five measurement positions using an SEM(“JSM-7500F” (trademark) provided by JEOL). The size of the measurementvisual field is set to 12 μm×15 μm and the observation magnification isset to 10000×.

By processing each of the five SEM images using image processingsoftware (Win Roof ver.7.4.5), the total number of the cubic boronnitride grains observed in the measurement visual field and theequivalent circle diameter of each of the cubic boron nitride grains arecalculated.

The ratio of the number of the cubic boron nitride grains each having anequivalent circle diameter of more than 0.5 μm is calculated using, as adenominator, the total number of the cubic boron nitride grains includedin each measurement visual field. The average value of the ratios of thenumbers of the cubic boron nitride grains each having an equivalentcircle diameter of more than 0.5 μm in the five measurement visualfields corresponds to the number-based ratio of the cubic boron nitridegrains each having an equivalent circle diameter of more than 0.5 μm inthe cubic boron nitride grains.

The ratio of the number of the cubic boron nitride grains each having anequivalent circle diameter of more than 2 μm is calculated using, as adenominator, the total number of the cubic boron nitride grains includedin each measurement visual field. The average value of the ratios of thenumbers of the cubic boron nitride grains each having an equivalentcircle diameter of more than 2 μm in the five measurement visual fieldscorresponds to the number-based ratio of the cubic boron nitride grainseach having an equivalent circle diameter of more than 2 μm in the cubicboron nitride grains.

It should be noted that in the measurement performed by the Applicant,as long as the total number of the cubic boron nitride grains and theequivalent circle diameter of each of the cubic boron nitride grains aremeasured in the same sample, results of measurement were notsubstantially varied even when measurement visual fields to be selectedwere changed and calculation was performed multiple times. It wasconfirmed that the results of measurement are not intentional even whena measurement visual field is set arbitrarily.

(Catalyst Elements)

In the cubic boron nitride sintered material according to the presentdisclosure, when the mass of the cubic boron nitride grains is assumedas 100 mass %, the total content of lithium, magnesium, calcium,strontium, beryllium, and barium in the cubic boron nitride grains isless than 0.001 mass %. In the cubic boron nitride sintered materialaccording to the present disclosure, the content of the catalystelements that promote the phase conversion from cubic boron nitride tohexagonal boron nitride is very small or no such catalyst elements arepresent, so that the phase conversion from cubic boron nitride tohexagonal boron nitride by the catalyst elements is less likely to occureven under the condition of pressure and temperature in the high-loadprocessing of hardened steel. Therefore, the tool employing the cubicboron nitride sintered material according to the present disclosure canhave excellent wear resistance and breakage resistance even in thehigh-load processing of hardened steel.

The lower limit of the total content of the catalyst elements in thecubic boron nitride grains is preferably 0 mass %. The upper limit ofthe total content of these catalyst elements is less than 0.001 mass %.The total content of these catalyst elements is preferably more than orequal to 0 mass % and less than 0.001 mass %.

The content of the catalyst elements in the cubic boron nitride grainscan be measured by high-frequency induction plasma emission spectrometry((ICP emission spectroscopy) with the use of a device “ICPS-8100”(trademark) provided by Shimadzu Corporation. Specifically, themeasurement can be performed in the following procedure.

First, the cubic boron nitride sintered material is immersed inhydrofluoric-nitric acid for 48 hours in a sealed container so as todissolve the binder phase in the hydrofluoric-nitric acid. Cubic boronnitride grains remaining in the hydrofluoric-nitric acid are subjectedto the high-frequency induction plasma emission spectrometry so as tomeasure the content of each of the catalyst elements. Thus, the totalcontent of the catalyst elements in the cubic boron nitride grains canbe measured.

<Binder Phase>

The binder phase included in the cubic boron nitride sintered materialaccording to the present disclosure includes at least one selected froma group consisting of one or more first compounds and a solid solutionoriginated from the first compounds, or includes at least one selectedfrom a group consisting of the one or more first compounds and the solidsolution originated from the first compounds and an aluminum compound,the one or more first compounds consisting of at least one element(hereinafter, also referred to as “first element”) selected from a groupconsisting of titanium (Ti), zirconium (Zr), hafnium (Hf), vanadium (V),niobium (Nb), tantalum (Ta), chromium (Cr), molybdenum (Mo), andtungsten (W), and at least one element selected from a group consistingof nitrogen (N), carbon (C), boron (B), and oxygen (0). Since each ofthe first compounds has high strength and toughness itself and can serveto firmly bind the cubic boron nitride grains, the strength of thesintered material is improved, with the result that the sinteredmaterial can have excellent wear resistance.

That is, the binder phase can have any one of the followingconfigurations (1) to (6):

(1) a binder phase including the first compound(s);

(2) a binder phase including the solid solution originated from thefirst compounds;

(3) a binder phase including the first compound(s) and the solidsolution originated from the first compounds;

(4) a binder phase including the first compound(s) and the aluminumcompound;

(5) a binder phase including the solid solution originated from thefirst compounds and the aluminum compound; and

(6) a binder phase including the first compound(s), the solid solutionoriginated from the first compounds, and the aluminum compound.

Examples of the first compound (nitride) composed of the firstelement(s) and nitrogen include titanium nitride (TiN), zirconiumnitride (ZrN), hafnium nitride (HfN), vanadium nitride (VN), niobiumnitride (NbN), tantalum nitride (TaN), chromium nitride (Cr₂N),molybdenum nitride (MoN), tungsten nitride (WN), titanium zirconiumnitride (TiZrN), titanium hafnium nitride (TiHfN), titanium vanadiumnitride (TiVN), titanium niobium nitride (TiNbN), titanium tantalumnitride (TiTaN), titanium chromium nitride (TiCrN), titanium molybdenumnitride (TiMoN), titanium tungsten nitride (TiWN), zirconium hafniumnitride (ZrHfN), zirconium vanadium nitride (ZrVN), zirconium niobiumnitride (ZrNbN), zirconium tantalum nitride (ZrTaN), zirconium chromiumnitride (ZrCrN), zirconium molybdenum nitride (ZrMoN), zirconiumtungsten nitride (ZrWN), hafnium vanadium nitride (HfVN), hafniumniobium nitride (HfNbN), hafnium tantalum nitride (HfTaN), hafniumchromium nitride (HfCrN), hafnium molybdenum nitride (HfMoN), hafniumtungsten nitride (HEWN), vanadium niobium nitride (VNbN), vanadiumtantalum nitride (VTaN), vanadium chromium nitride (VCrN), vanadiummolybdenum nitride (VMoN), vanadium tungsten nitride (VWN), niobiumtantalum nitride (NbTaN), niobium chromium nitride (NbCrN), niobiummolybdenum nitride (NbMoN), niobium tungsten nitride (NbWN), tantalumchromium nitride (TaCrN), tantalum molybdenum nitride (TaMoN), tantalumtungsten nitride (TaWN), chromium molybdenum nitride (CrMoN), chromiumtungsten nitride (CrWN), and molybdenum tungsten nitride (MoWN).

Examples of the first compound (carbide) composed of the firstelement(s) and carbon include titanium carbide (TiC), zirconium carbide(ZrC), hafnium carbide (HfC), vanadium carbide (VC), niobium carbide(NbC), tantalum carbide (TaC), chromium carbide (Cr₂C), molybdenumcarbide (MoC), tungsten carbide (WC), titanium zirconium carbide(TiZrC), titanium hafnium carbide (TiHfC), titanium vanadium carbide(TiVC), titanium niobium carbide (TiNbC), titanium tantalum carbide(TiTaC), titanium chromium carbide (TiCrC), titanium molybdenum carbide(TiMoC), titanium tungsten carbide (TiWC), zirconium hafnium carbide(ZrHfC), zirconium vanadium carbide (ZrVC), zirconium niobium carbide(ZrNbC), zirconium tantalum carbide (ZrTaC), zirconium chromium carbide(ZrCrC), zirconium molybdenum carbide (ZrMoC), zirconium tungstencarbide (ZrWC), hafnium vanadium carbide (HfVC), hafnium niobium carbide(HfNbC), hafnium tantalum carbide (HfTaC), hafnium chromium carbide(HfCrC), hafnium molybdenum carbide (HfMoC), hafnium tungsten carbide(HfWC), vanadium niobium carbide (VNbC), vanadium tantalum carbide(VTaC), vanadium chromium carbide (VCrC), vanadium molybdenum carbide(VMoC), vanadium tungsten carbide (VWC), niobium tantalum carbide(NbTaC), niobium chromium carbide (NbCrC), niobium molybdenum carbide(NbMoC), niobium tungsten carbide (NbWC), tantalum chromium carbide(TaCrC), tantalum molybdenum carbide (TaMoC), tantalum tungsten carbide(TaWC), chromium molybdenum carbide (CrMoC), chromium tungsten carbide(CrWC), and molybdenum tungsten carbide (MoWC).

Examples of the first compound (carbonitride) composed of the firstelement(s), carbon, and nitrogen include titanium carbonitride (TiCN),zirconium carbonitride (ZrCN), and hafnium carbonitride (HfCN).

Examples of the first compound (boride) composed of the first element(s)and boron include titanium boride (TiB₂), zirconium boride (ZrB₂),hafnium boride (HfB₂), vanadium boride (VB₂), niobium boride (NbB₂),tantalum boride (TaB₂), chromium boride (CrB₂), molybdenum boride(MoB₂), and tungsten boride (WB).

Examples of the first compound (oxide) composed of the first element(s)and oxygen include titanium oxide (TiO₂), zirconium oxide (ZrO₂),hafnium oxide (HfO₂), vanadium oxide (V₂O₅), niobium oxide (Nb₂O₅),tantalum oxide (Ta₂O₅), chromium oxide (Cr₂O₃), molybdenum oxide (MoO₃),and tungsten oxide (WO₃).

Examples of the first compound (oxynitride) composed of the firstelement(s), nitrogen, and oxygen include titanium oxynitride (TiON),zirconium oxynitride (ZrON), hafnium oxynitride (HfON), vanadiumoxynitride (VON), niobium oxynitride (NbON), tantalum oxynitride (TaON),chromium oxynitride (CrON), molybdenum oxynitride (MoON), and tungstenoxynitride (WON).

One of the first compounds may be solely used or two or more of thefirst compounds may be used in combination.

The binder phase can include the solid solution originated from thefirst compounds. Here, the solid solution originated from the firstcompounds refers to a state in which two or more of the first compoundsare dissolved in the crystal structures of the compounds, and refers toan interstitial solid solution or a substitutional solid solution.

Examples of the aluminum compound include titanium aluminum nitride(TiAlN, Ti₂AlN, or Ti₃AlN), titanium aluminum carbide (TiAlC, Ti₂AlC, orTi₃AlC), titanium aluminum carbonitride (TiAlCN, Ti₂AlCN, or Ti₃AlCN),aluminum boride (AlB₂), aluminum oxide (Al₂O₃), AlN (aluminum nitride),and SiAlON (sialon).

One of the aluminum compounds may be used or two or more of the aluminumcompounds may be used in combination.

The binder phase can consist only of one or more selected from a groupconsisting of the first compound(s) and the solid solution of the firstcompounds. Further, the binder phase can consist of: more than or equalto 99.9 volume % of one or more selected from the group consisting ofthe first compound(s) and the solid solution of the first compounds; anda remainder.

The binder phase can consist only of one or more selected from the groupconsisting of the first compound(s) and the solid solution of the firstcompounds, and the aluminum compound(s). Further, the binder phase canconsist of: more than or equal to 99.9 volume % of the total of one ormore selected from the group consisting of the first compound(s) and thesolid solution of the first compounds and the aluminum compound(s); anda remainder.

Here, the remainder corresponds to an inevitable impurity in the binderphase. The content ratio of the inevitable impurity in the cubic boronnitride sintered material is preferably less than or equal to 0.1 mass%.

The composition of the binder phase can be measured using an X-raydiffraction method. A specific measurement method is as follows.

First, the cubic boron nitride sintered material is cut by a diamondgrindstone electrodeposited wire or the like to expose measurementpositions, and the cross section thereof is polished. When the cubicboron nitride sintered material is used as a portion of a tool, theportion of the cubic boron nitride sintered material is cut out by thediamond grindstone electrodeposited wire or the like, and the crosssection of the cut-out portion is polished. Five measurement positionsare arbitrarily set on the polished surface.

An X-ray diffractometer (“MiniFlex600” (trademark) provided by Rigaku)is used to obtain an X-ray diffraction spectrum of the polished surface.Conditions for the X-ray diffractometer on this occasion are as follows.

Characteristic X ray: Cu-Kα (wavelength of 1.54 Å)

Tube voltage: 45 kV

Tube current: 40 mA

Filter: multilayer mirror

Optical system: concentration method

X-ray diffraction method: θ-2 θ method.

Based on the obtained X-ray diffraction spectrum, the composition of thebinder phase is identified. <X-Ray Diffraction Spectrum>

In the cubic boron nitride sintered material according to the presentdisclosure, in an X-ray diffraction spectrum of the cubic boron nitridesintered material, a relation of the following formula I is preferablyindicated:

(IA+IB+IC)/ID≤0.05  formula I, where

IA represents a peak intensity originated from compressed hexagonalboron nitride, IB represents a peak intensity originated from hexagonalboron nitride, IC represents a peak intensity originated from wurtzitetype boron nitride, and ID represents a peak intensity originated fromcubic boron nitride.

The compressed hexagonal boron nitride, the hexagonal boron nitride, thewurtzite type boron nitride and the cubic boron nitride all have similardegrees of electron densities. Therefore, the ratio of peak intensityIA, peak intensity IB, peak intensity IC, and peak intensity ID in theX-ray diffraction spectrum can be regarded as the volume ratio of thecompressed hexagonal boron nitride, the hexagonal boron nitride, thewurtzite type boron nitride, and the cubic boron nitride in the cubicboron nitride sintered material.

When peak intensity IA, peak intensity IB, peak intensity IC, and peakintensity ID satisfy the above-described relation of formula I, thevolume ratio of the compressed hexagonal boron nitride, the hexagonalboron nitride, and the wurtzite type boron nitride in the cubic boronnitride sintered material is much smaller than the volume ratio of thecubic boron nitride. Therefore, the influence on the cubic boron nitridesintered material by the compressed hexagonal boron nitride, thehexagonal boron nitride, and the wurtzite type boron nitride, i.e.,decreased strength and toughness, is very little, with the result thatthe cubic boron nitride sintered material can have excellent wearresistance and breakage resistance.

The following specifically describes a method of measuring each of peakintensity IA originated from the compressed hexagonal boron nitride,peak intensity IB originated from the hexagonal boron nitride, peakintensity IC originated from the wurtzite type boron nitride, and peakintensity ID originated from the cubic boron nitride.

First, the cubic boron nitride sintered material is cut by a diamondgrindstone electrodeposited wire or the like so as to expose measurementpositions, and the cross section thereof is polished. When the cubicboron nitride sintered material is used as a portion of a tool, theportion of the cubic boron nitride sintered material is cut out by thediamond grindstone electrodeposited wire or the like, and the crosssection of the cut-out portion is polished. Five measurement positionsare arbitrarily set on the polished surface.

An X-ray diffractometer (“MiniFlex600” (trademark) provided by Rigaku)is used to obtain an X-ray diffraction spectrum of the polished surface.Conditions for the X-ray diffractometer on this occasion are as follows.

Characteristic X ray: Cu-Kα (wavelength of 1.54 Å)

Tube voltage: 45 kV

Tube current: 40 mA

Filter: multilayer mirror

Optical system: concentration method

X-ray diffraction method: θ-2 θ method.

In the obtained X-ray diffraction spectrum, the following peak intensityIA, peak intensity IB, peak intensity IC, and peak intensity ID aremeasured.

Peak intensity IA: the peak intensity of the compressed hexagonal boronnitride obtained by excluding background from a peak intensity at adiffraction angle 2 θ=about 28.5°.

Peak intensity IB: the peak intensity of the hexagonal boron nitrideobtained by excluding background from a peak intensity at a diffractionangle 2 θ=about 41.6°.

Peak intensity IC: the peak intensity of the wurtzite type boron nitrideobtained by excluding background from a peak intensity at a diffractionangle 2 θ=about 40.8°.

Peak intensity ID: the peak intensity of the cubic boron nitrideobtained by excluding background from a peak intensity at a diffractionangle 2 θ=about 43.5°.

At each of the five measurement positions, peak intensity IA, peakintensity IB, peak intensity IC, and peak intensity ID are measured toobtain the value of (IA+IB+IC)/ID. The average value of the values of(IA+IB+IC)/ID at the five measurement positions corresponds to(IA+IB+IC)/ID in the cubic boron nitride sintered material.

It should be noted that in the measurement performed by the Applicant,as long as peak intensity IA, peak intensity IB, peak intensity IC, andpeak intensity ID are measured in the same sample, results ofmeasurement were not substantially varied even when measurementpositions to be selected were changed and calculation was performedmultiple times. It was confirmed that the results of measurement are notintentional even when a measurement position is set arbitrarily.

<Application>

The cubic boron nitride sintered material according to the presentdisclosure is suitably used for a cutting tool, a wear-resistant tool, agrinding tool, or the like.

Each of the cutting tool, the wear-resisting tool and the grinding toolemploying the cubic boron nitride sintered material according to thepresent disclosure may be entirely constituted of the cubic boronnitride sintered material, or only a portion thereof (for example, acutting edge portion in the case of the cutting tool) may be constitutedof the cubic boron nitride sintered material. Moreover, a coating filmmay be formed on a surface of each of the tools.

Examples of the cutting tool include a drill, an end mill, an indexablecutting insert for drill, an indexable cutting insert for end mill, anindexable cutting insert for milling, an indexable cutting insert forturning, a metal saw, a gear cutting tool, a reamer, a tap, a cuttingbite, and the like.

Examples of the wear-resistant tool include a die, a scriber, a scribingwheel, a dresser, and the like. Examples of the grinding tool include agrinding stone and the like.

Second Embodiment Method of Producing Cubic Boron Nitride SinteredMaterial

A method of producing a cubic boron nitride sintered material accordingto the present disclosure is a method of producing the cubic boronnitride sintered material according to the first embodiment andincludes: a first step of obtaining a powder mixture by mixing ahexagonal boron nitride powder and a binder powder; and a second step ofobtaining the cubic boron nitride sintered material by sintering thepowder mixture by increasing a pressure to more than or equal to 8 GPaand less than or equal to 20 GPa, increasing a temperature to more thanor equal to 2300° C. and less than or equal to 2500° C., and holding,for more than or equal to 30 minutes and less than 90 minutes, thepowder mixture at maximum pressure and maximum temperature reached byincreasing the pressure and the temperature.

<First Step>

First, a hexagonal boron nitride powder and a binder powder areprepared. The purity (content ratio of the hexagonal boron nitride) ofthe hexagonal boron nitride powder is preferably more than or equal to98.5%, is more preferably more than or equal to 99%, and is mostpreferably 100%. Each of the particle sizes of the hexagonal boronnitride powder is not particularly limited, and can be more than orequal to 0.1 μm and less than or equal to 10 μm, for example.

The binder powder is selected in accordance with the composition of theintended binder phase. Specifically, particles each composed of thefirst compound(s) described in the binder phase of the first embodimentcan be used. In addition to the particles composed of the firstcompound(s), particles each composed of the aluminum compound(s) can beused. Each of the particle sizes of the binder powder is notparticularly limited, and can be more than or equal to 0.1 μm and lessthan or equal to 10 μm, for example.

Next, the hexagonal boron nitride powder and the binder powder are mixedto obtain a powder mixture. The mixing ratio of the hexagonal boronnitride powder and the binder powder is adjusted such that the ratio ofthe cubic boron nitride grains in the finally obtained cubic boronnitride sintered material becomes more than or equal to 40 volume % andless than or equal to 85 volume %.

For the mixing, for example, a mixing device such as a ball mill or anattritor can be used. A mixing time is, for example, more than or equalto about 5 hours and less than or equal to about 24 hours.

In the powder mixture thus obtained, the hexagonal boron nitride powdercan include boron oxide generated by influence of surface oxidationduring the mixing, can include moisture, or can include an adsorptiongas. These impurities inhibit direct conversion from hexagonal boronnitride to cubic boron nitride. Otherwise, these impurities act ascatalysts to cause grain growth, thereby weakening the binding betweenthe cubic boron nitride grains. Therefore, it is preferable to removethe impurities by performing high temperature purification treatment.For example, the boron oxide or the adsorption gas can be removed byperforming heat treatment onto the powder mixture under a condition ofmore than or equal to 2050° C. in a nitrogen gas or under a condition ofmore than or equal to 1650° C. in vacuum. The powder mixture thusobtained includes a very small amount of impurities and is suitable fordirect conversion from hexagonal boron nitride to cubic boron nitride.

(Second Step)

In the second step, the cubic boron nitride sintered material isobtained by sintering the powder mixture by increasing a pressure tomore than or equal to 8 GPa and less than or equal to 20 GPa, increasinga temperature to more than or equal to 2300° C. and less than or equalto 2500° C., and holding, for more than or equal to 30 minutes and lessthan 90 minutes, the powder mixture at maximum pressure and maximumtemperature reached by increasing the pressure and the temperature. Onthis occasion, the hexagonal boron nitride is converted directly intothe cubic boron nitride. Further, the powder mixture is sintered at thesame time as the conversion from hexagonal boron nitride to cubic boronnitride, thereby obtaining the cubic boron nitride sintered material.

In the second step, the conversion from hexagonal boron nitride to cubicboron nitride is performed directly without using a catalyst element. Inorder to attain this, the pressure and temperature in the sinteringcondition for the powder mixture need to be higher than the pressure andtemperature in the conventional sintering condition in which a catalystelement is used.

EXAMPLES

The following describes the present embodiment more specifically by wayof examples. However, the present embodiment is not limited by theseexamples.

Example 1

<Production of Cubic Boron Nitride Sintered Material>

[Samples No. 1 to No. 6 and Samples No. 13 to No. 22]

Each of cubic boron nitride sintered materials of samples No. 1 to No. 6and samples No. 13 to No. 22 was produced by the following productionmethod.

(First Step)

First, a hexagonal boron nitride powder (indicated as “hBN powder” inTable 1) having an average particle size of 10 μm and a binder powderhaving a composition shown in the column “Source Material Powders” of“First Step” of Table 1 were prepared as starting materials (sourcematerials). For example, in sample No. 1, a TiN powder was prepared asthe binder powder.

The mixing ratio of the hexagonal boron nitride powder and the binderpowder was adjusted such that the ratio of the cubic boron nitridegrains in the finally obtained cubic boron nitride sintered materialbecame the ratio described in the column “cBN Grains (Volume %)” of the“Cubic Boron Nitride Sintered Material” in Table 1. When two or morebinder powders were used, each of the mass ratios of the binder powdersis described in parentheses subsequent to the composition of the binderpowder. For example, in sample No. 17, it is indicated that 85 mass % ofTiN_(0.5) powder and 15 mass % of Al powder are included in the binderpowder.

The hexagonal boron nitride powder and the binder powder were mixed for5 hours using a ball mill. Thus, a powder mixture was obtained. Thepowder mixture was subjected to heat treatment at a temperature of 2050°C. under a nitrogen atmosphere, thereby removing impurities (hightemperature purification treatment).

(Second Step)

The powder mixture having been through the high temperature purificationtreatment was introduced into a capsule composed of tantalum, and washeld at pressure, temperature, and time shown in the columns “Pressure(GPa)”, “Temperature (° C.)”, and “Time (min)” of “Second Step” in Table1 using an ultra-high pressure and high temperature generationapparatus, thereby obtaining the cubic boron nitride sintered material.

[Samples No. 7 to No. 12]

Each of cubic boron nitride sintered materials of samples No. 7 to No.12 was produced by the following production method.

(First Step)

First, a cubic boron nitride powder (indicated as “cBN Powder” inTable 1) having an average particle size of 1μm and a binder powderhaving a composition shown in the column “Source Material Powders” of“First Step” of Table 1 were prepared as starting materials (sourcematerials). The cubic boron nitride powder is produced by a conventionalmethod using a catalyst.

The cubic boron nitride powder and the binder powder were blended in thefollowing volume ratio: the cBN powder:the binder powder=55:45. Further,the cubic boron nitride powder and the binder powder were mixed for 5hours using a ball mill. Thus, a powder mixture was obtained. The powdermixture was subjected to heat treatment at a temperature of 2050° C.under a nitrogen atmosphere, thereby removing impurities (hightemperature purification treatment).

(Second Step)

The powder mixture having been through the high temperature purificationtreatment was introduced into a capsule composed of tantalum, and washeld at pressure, temperature, and time shown in the columns “Pressure(GPa)”, “Temperature (° C.)”, and “Time (min)” of “Second Step” in Table1 using an ultra-high pressure and high temperature generationapparatus, thereby obtaining the cubic boron nitride sintered material.

TABLE 1 Cubic Boron Nitride Sintered Material Number-Based Ratio of cBNGrains Production Conditions Equivalent Equivalent First Step CircleCircle Source Second Step cBN Binder Diameter of Diameter of SampleMaterial Pressure Temperature Time Grains Phase More Than More Than No.Powders (GPa) (° C.) (min) (Volume %) (Volume %) 0.5 μm (%) 2 μm (%)  1hBN, TiN 14 2300 40 35 65 85 22   2-1 hBN, TiN 14 2300 40 40 60 85 22  2-2 hBN, TiN 14 2300 40 45 55 85 22  3 hBN, TiN 14 2300 40 55 45 85 22 4 hBN, TiN 14 2300 40 75 25 85 22  5 hBN, TiN 14 2300 40 85 15 85 22  6hBN, TiN 14 2300 40 90 10 85 22  7 cBN, TiN 5 1250 20 55 45 85 22  8cBN, TiN 5 1250 20 55 45 85 22  9 cBN, TiN 5 1250 20 55 45 85 22 10 cBN,TiN 5 1250 20 55 45 85 22 11 cBN, TiN 5 1250 20 55 45 85 22 12 cBN, TiN5 1250 20 55 45 85 22 13 hBN, TiN 10 2200 20 55 45 38 5 14 hBN, TiN 102400 35 55 45 55 15 15 hBN, TiN 14 2400 60 55 45 90 45 16 hBN, TiN 142400 90 55 45 96 70 17 hBN, TiN_(0.5)(85), 14 2300 40 55 45 85 22 Al(15)18 hBN, TiC 14 2300 40 55 45 85 22 19 hBN, TiC_(0.6)(50), 14 2300 40 5545 85 22 HfC(50) 20 hBN, TiN(50), 14 2300 40 55 45 85 22 TiC(50) 21 hBN,Al₂O₃(90), 14 2300 40 55 45 85 22 ZrN(5), ZrC(5) 22 hBN, TiN_(0.7) 142300 40 55 45 85 22 Cubic Boron Nitride Sintered Material EvaluationComposition Contents of Catalyst Elements Flank Wear Sample of Binder(IA + IB + in cBN Grains (Mass %) Amount No. Phase IC)/ID Li Mg Ca Sr BaBe (mm)  1 TiN — — — — — — — Breakage   2-1 TiN — — — — — — — 0.072 FineChipping   2-2 TiN — — — — — — — 0.046  3 TiN — — — — — — — 0.062  4 TiN— — — — — — — 0.079  5 TiN — — — — — — — 0.092  6 TiN — — — — — — —0.121  7 TiN —  0.004 — — — — — 0.119  8 TiN — 0.01  0.002 — — — — 0.128 9 TiN — — 0.02 0.008 — — — 0.132 10 TiN —  0.008 0.03 — 0.01 — — 0.13811 TiN — 0.03 0.01 — — 0.03 — 0.142 12 TiN — 0.01  0.002 0.05  0.04 —0.01 0.182 13 TiN — — — — — — — 0.111 14 TiN — — — — — — — 0.082 15 TiN— — — — — — — 0.072 16 TiN — — — — — — — Breakage 17 TiN, TiB₂, — — — —— — — 0.065 Ti₂AiN 18 TiC — — — — — — — 0.053 19 TiC, TiB₂, — — — — — —— 0.068 HfB₂, HfC 20 TiN, TiC, — — — — — — — 0.058 TiB₂ 21 Al₂O₃, ZrN, —— — — — — — 0.083 ZrC 22 TiN, TiB₂ — — — — — — — 0.070

<Measurement>

(Composition of Cubic Boron Nitride Sintered Material)

The composition of the cubic boron nitride sintered material of each ofthe samples (the content ratio of the cubic boron nitride and thecontent ratio of the binder phase) was measured by image analysis on anSEM reflected electron image. A specific measurement method is indicatedin the first embodiment, and therefore will not be described repeatedly.Results are shown in the columns “cBN Grains (Volume %)” and “BinderPhase (Volume %)” of the “Cubic Boron Nitride Sintered Material” inTable 1.

(Composition of Binder Phase)

The composition of the binder phase of each sample was measured using anX-ray diffraction method. A specific measurement method is indicated inthe first embodiment, and therefore will not be described repeatedly.Results are shown in the column “Composition of Binder Phase” in Table1.

(Grain Sizes of Cubic Boron Nitride Grains)

The grain sizes of the cubic boron nitride grains of each of the sampleswere measured by image analysis on an SEM reflected electron image, andthe number-based ratio of cubic boron nitride grains each having anequivalent circle diameter of more than 0.5 μm and the number-basedratio of cubic boron nitride grains each having an equivalent circlediameter of more than 2 μm were calculated. A specific measurementmethod is indicated in the first embodiment, and therefore will not bedescribed repeatedly. Results are shown in the columns “EquivalentCircle Diameter of More Than 0.5 μm (%)” and “Equivalent Circle Diameterof More Than 2 μm (%)” of “Number-Based Ratio of cBN Grains” in Table 1.

(Content of Catalyst Elements)

Types and contents of the catalyst elements in the cubic boron nitridegrains of each of the samples were measured by ICP emissionspectrometry. A specific measurement method is indicated in the firstembodiment, and therefore will not be described repeatedly. Results areshown in the column “Contents of Catalyst Elements in cBN Grains (Mass%)” in Table 1. It should be noted that when “-” is indicated in aresult, it is indicated that the catalyst elements were not detected andthe content thereof is less than the detection limit (0.001 mass %).

(X-Ray Diffraction Spectrum)

X-ray diffraction measurement was performed on the cubic boron nitridegrains of each of the samples to obtain an X-ray diffraction spectrum. Aspecific measurement method is indicated in the first embodiment, andtherefore will not be described repeatedly. Based on the X-raydiffraction spectrum, peak intensity IA originated from the compressedhexagonal boron nitride, peak intensity IB originated from the hexagonalboron nitride, peak intensity IC originated from the wurtzite type boronnitride, and peak intensity ID originated from the cubic boron nitridewere measured to obtain the value of (IA+IB+IC)/ID. Results are shown inthe column “(IA+IB+IC)/ID” in Table 1. It should be noted that when “-”is indicated in a result, it is indicated that none of peak intensity IAoriginated from the compressed hexagonal boron nitride, peak intensityIB originated from the hexagonal boron nitride, and peak intensity ICoriginated from the wurtzite type boron nitride was detected and onlythe peak originated from the cubic boron nitride was detected. That is,when “-” is indicated in a result, it is indicated that (IA+IB+IC)/ID=0.

<Evaluation>

(Production of Cutting Tool and Evaluation on Cutting Performance)

A tool having a tool shape SNMN120408 was produced using the cubic boronnitride sintered material of each of the samples as its cutting edge. Acutting test was performed using the tool under the following cuttingconditions to evaluate wear resistance and breakage resistance.

(Cutting Conditions)

Cutting method: dry cutting

Workpiece: bearing steel SUJ2 round bar (hardness HRc61)

Cutting rate: 170 m/min

Feed: 0.12 mm/rev.

Depth of cut: ap=0.2 mm

Evaluation method: flank wear amount (unit: mm) when cutting wasperformed for 10 minutes.

The above-described cutting conditions correspond to the high-loadprocessing of hardened steel. It is indicated that as the flank wearamount is smaller, the wear resistance is higher and the cuttingperformance is more excellent. Results are shown in the column “FlankWear Amount (mm)” in Table 1. It should be noted that “Breakage” isindicated when breakage of the tool occurred within 10 minutes afterstarting the cutting.

<Analysis>

Each of the production conditions and the obtained cubic boron nitridesintered materials of samples No. 2-1, No. 2-2, No. 3 to No. 5, No. 14,No. 15, and No. 17 to No. 22 corresponds to an example of the presentdisclosure. Each of the tools employing these cubic boron nitridesintered materials exhibited excellent cutting performance even in thehigh-load processing of hardened steel.

In the cubic boron nitride sintered material obtained by the productionmethod of sample No. 1, the content ratio of the cubic boron nitridegrains was 35 volume %, which corresponds to a comparative example.Therefore, the production condition of sample No. 1 also corresponds tothe comparative example. Breakage of the tool employing the cubic boronnitride sintered material occurred within 10 minutes after starting thecutting. This is presumably due to the following reason: since the ratioof the cubic boron nitride grains was less than 40 volume %, the ratioof the binder phase inferior in strength was large in the cubic boronnitride sintered material, with the result that the hardness of thecubic boron nitride sintered material was decreased.

In the cubic boron nitride sintered material obtained by the productionmethod of sample No. 6, the content ratio of the cubic boron nitridegrains was 90 volume %, which corresponds to a comparative example.Therefore, the production condition of sample No. 6 also corresponds tothe comparative example. The tool employing the cubic boron nitridesintered material had a large flank wear amount and inferior wearresistance as compared with those of the example of the presentdisclosure. This is presumably due to the following reason: since theratio of the cubic boron nitride grains was more than 85 volume %, theratio of the binder phase that maintains binding between the cubic boronnitride grains was small, with the result that the cubic boron nitridegrains were likely to fall to facilitate progress of wear.

Each of the production conditions of samples No. 7 to No. 12 employs acubic boron nitride powder produced by a conventional method using acatalyst and corresponds to a comparative example. In each of thesintered materials of samples No. 7 to No. 12, the content of thecatalyst elements in the cubic boron nitride grains was more than orequal to 0.001 mass %, which corresponds to a comparative example.

Each of tools employing these cubic boron nitride sintered materials hada large flank wear amount and inferior wear resistance as compared withthose of the example of the present disclosure. This is presumably dueto the following reason: since the catalyst elements present in thecubic boron nitride grains promoted phase conversion from cubic boronnitride to hexagonal boron nitride under a condition of pressure andtemperature in the vicinity of a contact point with a workpiece duringthe processing, the hardness and thermal conductivity were decreased atthe cutting edge portion region, with the result that the wearresistance was decreased.

The production condition of sample No. 13 is such that a temperature is2200° C. and a holding time is 20 minutes in the second step, andcorresponds to a comparative example. In the cubic boron nitridesintered material of sample No. 13, the number-based ratio of the cubicboron nitride grains each having an equivalent circle diameter of morethan 0.5 μm was 38%, which corresponds to a comparative example. Thetool employing the cubic boron nitride sintered material had a largeflank wear amount and inferior wear resistance as compared with those ofthe example of the present disclosure. This is presumably due to thefollowing reason: since the content ratio of the cubic boron nitridegrains each having an equivalent circle diameter of more than 0.5 μm wassmall, the ratio of the fine grains each having an equivalent circlediameter of less than or equal to 0.5 μm was large, with the result thatthe toughness and thermal conductivity were decreased to result indecreased wear resistance.

The production condition of sample No. 16 was such that a holding timewas 90 minutes in the second step, and corresponds to a comparativeexample. In the cubic boron nitride sintered material of sample No. 16,the cubic boron nitride grains included, on number basis, 70% of cubicboron nitride grains each having an equivalent circle diameter of morethan 2 μm and the cubic boron nitride sintered material of sample No. 16corresponds to a comparative example. In the tool employing the cubicboron nitride sintered material, breakage occurred within 10 minutesafter starting the cutting. This is presumably due to the followingreason: since the content ratio of the cubic boron nitride grains eachhaving an equivalent circle diameter of more than 2 μm was large, thestrength was decreased to result in decreased breakage resistance.

Example 2

<Production of Cubic Boron Nitride Sintered Material>

[Samples No. 3, No. 8, and No. 23 to No. 26]

Cubic boron nitride sintered materials of samples No. 3 and No. 8 wereproduced in the same manner as in Example 1. In each of samples No. 23to No. 26, a cubic boron nitride sintered material was produced in thesame manner as in sample No. 1 except that the conditions in the firststep and the second step were changed to those shown in the columns“First Step” and “Second Step” in Table 2.

TABLE 2 Cubic Boron Nitride Sintered Material Number-Based ProductionConditions Ratio of cBN Grains First Step Equivalent Equivalent SourceSecond Step cBN Binder Circle Diameter Circle Diameter Sample MaterialPressure Temperature Time Grains Phase of More Than of More Than No.Powders (GPa) (° C.) (min) (Volume %) (Volume %) 0.5 μm(%) 2 μm(%) 3hBN, TiN 14 2300 40 55 45 85 22 8 cBN, TiN 5 1250 20 55 45 85 22 23 hBN,TiN 15 2300 40 55 45 85 22 24 hBN, TiN 16 2300 40 55 45 85 22 25 hBN,TiN 17.5 2300 40 55 45 85 22 26 hBN, TiN 19 2300 40 55 45 85 22 CubicBoron Nitride Sintered Material Evaluation Composition Contents ofCatalyst Elements Cutting Time Sample of Binder (IA + IB + in cBN Grains(Mass %) until Breakage No. Phase IC)/ID Li Mg Ca Sr Ba Be (min) 3 TiN —— — — — — — 30 8 TiN — 0.01 0.002 — — — — 9 23 TiN 0.012 — — — — — — 2224 TiN 0.042 — — — — — — 21 25 TiN 0.06 — — — — — — 15 26 TiN 0.08 — — —— — — 14

<Measurement>

In each of the samples, the composition of the cubic boron nitridesintered material (the content of the cubic boron nitride and thecontent of the binder phase), the composition of the binder phase, thegrain sizes of the cubic boron nitride grains, the content of thecatalyst elements, and the X-ray diffraction spectrum were measured inthe same manner as in Example 1. Results are shown in Table 2.

<Evaluation>

(Production of Cutting Tool and Evaluation on Cutting Performance) Atool having a tool shape SNMN120408 was produced using the cubic boronnitride sintered material of each of the samples as its cutting edge. Acutting test was performed using the tool under the following cuttingconditions to evaluate breakage resistance.

(Cutting Conditions)

Cutting method: dry cutting

Workpiece: carburized hardened steel, SCr420H, 4U groove (hardnessHRc62)

Cutting rate: 170 m/min

Feed: 0.16 mm/rev.

Depth of cut: ap=0.2 mm

Evaluation method: cutting time until occurrence of breakage (min).

The above-described cutting conditions correspond to the high-loadprocessing of hardened steel. It is indicated that as the cutting timeuntil occurrence of breakage is longer, the breakage resistance ishigher and the cutting performance is more excellent. Results are shownin the column “Cutting Time until Breakage (min)” in Table 2.

<Analysis>

Each of the production conditions and the obtained cubic boron nitridesintered materials of samples No. 3 and No. 23 to No. 26 correspond toan example of the present disclosure. In each of the tools employingthese cubic boron nitride sintered materials, the time until occurrenceof breakage was long and excellent cutting performance was exhibitedeven in the high-load processing of hardened steel. Among them, in eachof the cubic boron nitride sintered materials of samples No. 3, No. 23and No. 24, the value of (IA+IB+IC)/ID was less than or equal to 0.05,and the cutting performance was very excellent.

The production condition of sample No. 8 employs a cubic boron nitridepowder produced by a conventional method using a catalyst andcorresponds to a comparative example. In the sintered material of sampleNo. 8, the content of the catalyst elements in the cubic boron nitridegrains was more than or equal to 0.001 mass %, which corresponds to acomparative example. In the tool employing the cubic boron nitridesintered material, the time until occurrence of breakage was short andbreakage resistance was inferior as compared with those in the exampleof the present disclosure. This is presumably due to the followingreason: the catalyst elements present in the cubic boron nitride grainspromoted phase conversion from cubic boron nitride to hexagonal boronnitride under a condition of pressure and temperature in the vicinity ofa contact point with the workpiece during the processing, with theresult that the hardness and thermal conductivity were decreased in thecutting edge portion region to result in decreased breakage resistance.

Heretofore, the embodiments and examples of the present disclosure havebeen illustrated, but it has been initially expected to appropriatelycombine the configurations of the embodiments and examples and modifythem in various manners.

The embodiments and examples disclosed herein are illustrative andnon-restrictive in any respect. The scope of the present invention isdefined by the terms of the claims, rather than the embodiments andexamples described above, and is intended to include any modificationswithin the scope and meaning equivalent to the terms of the claims.

1. A cubic boron nitride sintered material comprising: more than orequal to 40 volume % and less than or equal to 85 volume % of cubicboron nitride grains; and a binder phase, wherein the binder phaseincludes at least one selected from a group consisting of one or morefirst compounds and a solid solution originated from the firstcompounds, or includes at least one selected from a group consisting ofthe one or more first compounds and the solid solution originated fromthe first compounds and an aluminum compound, the one or more firstcompounds consisting of at least one element selected from a groupconsisting of titanium, zirconium, hafnium, vanadium, niobium, tantalum,chromium, molybdenum, and tungsten, and at least one element selectedfrom a group consisting of nitrogen, carbon, boron, and oxygen, thecubic boron nitride grains include, on number basis, more than or equalto 50% of cubic boron nitride grains each having an equivalent circlediameter of more than 0.5 μm, and includes, on number basis, less thanor equal to 50% of cubic boron nitride grains each having an equivalentcircle diameter of more than 2 μm, and when a mass of the cubic boronnitride grains is assumed as 100 mass %, a total content of lithium,magnesium, calcium, strontium, beryllium, and barium in the cubic boronnitride grains is less than 0.001 mass %.
 2. The cubic boron nitridesintered material according to claim 1, wherein in an X-ray diffractionspectrum of the cubic boron nitride sintered material, a relation of thefollowing formula I is indicated:(IA+IB+IC)/ID≤0.05  formula I, where IA represents a peak intensityoriginated from compressed hexagonal boron nitride, IB represents a peakintensity originated from hexagonal boron nitride, IC represents a peakintensity originated from wurtzite type boron nitride, and ID representsa peak intensity originated from cubic boron nitride.
 3. A method ofproducing the cubic boron nitride sintered material according to claim1, the method comprising: obtaining a powder mixture by mixing ahexagonal boron nitride powder and a binder powder; and obtaining thecubic boron nitride sintered material by sintering the powder mixture byincreasing a pressure to more than or equal to 8 GPa and less than orequal to 20 GPa, increasing a temperature to more than or equal to 2300°C. and less than or equal to 2500° C., and holding, for more than orequal to 30 minutes and less than 90 minutes, the powder mixture atmaximum pressure and maximum temperature reached by increasing thepressure and the temperature.
 4. A method of producing the cubic boronnitride sintered material according to claim 2, the method comprising:obtaining a powder mixture by mixing a hexagonal boron nitride powderand a binder powder; and obtaining the cubic boron nitride sinteredmaterial by sintering the powder mixture by increasing a pressure tomore than or equal to 8 GPa and less than or equal to 20 GPa, increasinga temperature to more than or equal to 2300° C. and less than or equalto 2500° C., and holding, for more than or equal to 30 minutes and lessthan 90 minutes, the powder mixture at maximum pressure and maximumtemperature reached by increasing the pressure and the temperature.